Single-photon sources are the core element of quantum cryptography. In the exchange of sensitive data such as e.g. online business transactions, they offer absolute interception protection based on the laws of quantum mechanics.
An ideal single-photon source (photon gun) is a component which emits a single photon after a trigger signal, and only then (on demand). The central element of a single-photon source is optimally a quantized system with discrete energy levels.
German Patent Application DE 10 2008 036 400 describes a single-photon source having a cylindrical cavity. The cavity comprises a first mirror and a second mirror and exhibits a longitudinal resonance frequency between the first and second mirrors. The single-photon source emits photons efficiently only if the quantum dot's radiation frequency corresponds to the cavity's longitudinal resonance frequency. As such, the quality factor of the cavity needs to be limited since a large quality factor would reduce the chance that the radiation frequency matches the cavity's longitudinal resonance frequency. In summary, the tolerance range of this type of single-photon source is small, and the fabrication yield is poor.